Chapter 27
Atmospheric Reactions of CH S Radicals Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 10, 2018 at 05:04:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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G. S. Tyndall and A. R. Ravishankara National Oceanic and Atmospheric Administration, Environmental Research Laboratory, R/E/AL2, Boulder, CO 80303 and CIRES, University of Colorado, Boulder, CO 80309 Rate coefficients have been measured for the reactions of C H S radicals with NO and O at 298 K. The rate coefficient for CH S + NO is (6.1 ± 0.7) x 10 cm molec s . NO was found to be the major product of the reaction, consistent with the mechanism C H S + NO ---> CH SO + NO. CH SO reacts withNO to give CH SO + NO. We were only able to assign an upper limit, ≤ 2.5 x 10 c m molec s , to the rate coefficient for the reaction of CH S with O . Despite this low rate coefficient this reaction could still be important in the marine troposphere. The impact of these rate coefficients on the production of SO and C H S O H in the atmosphere is discussed. Tropospheric oxidation of dimethyl sulfide, methyl mercaptan and dimethyl disulfide, the major organo-sulfur compounds released into the atmosphere, is initiated by reaction with OH, or to a lesser extent, NOj (1.2). The products of these reactions are not well known, and the details of the subsequent reactions leading to completely or partially oxidized sulfur species are obscure. The knowledge that we do have on this oxidation sequence is primarily derived from indirect studies, most of which used elevated levels (>90 ppb) of NO^ (3-5). For example, all these studies agree that in the oxidation of CH3SCH3 (the most abundant of the organic sulmr compounds) S 0 and CH3SO3H (methane sulfonic acid, MSA) are the major observed products. However, considerable discrepancies exist in the yield of S 0 relative to MSA measured by various laboratories, and the atmospheric observations of S 0 and MSA (£) do not agree with the product distributions seen in the laboratory. At present the exact details of the mechanisms leading to each of the products are far from well understood (7 8). However, it seems likely that oxidation of the common naturally-occurring organo-sulfur compounds (dimethyl sulfide, methyl mercaptan and dimethyl disulfide) proceeds, at least in part, through formation of the methyl thiyl radical, C H S (4.9-11). That is the main reason for carrying out direct studies on CH3S radical reactions where the individual reactions are isolated. Studies using continuous photolysis to produce C H S followed by end product analysis suggest that the reactivity of CH3S with 0 is low, with a rate 3
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0097-6156/89/0393-0450$06.00A) 1989 American Chemical Society
Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
27. TYNDALL AND RAVISHANKARA
Atmospheric Reactions ofCHjS Radicals 451
coefficient several orders of magnitude smaller than that with NO or N 0 (3.4.7). Recent measurements by lialla et al. (12), who followed CH3S directly in time-resolved experiments, support this conclusion. However, results obtained on the oxidation of CH3S radicals in the complete absence of N O indicate greater than 80% conversion of the sulfur to S 0 (3.13.14). indicating the possible occurrence of the CH3S + 0 reaction. Because of these uncertainties in the CH3S oxidation scheme, we have initiated a systematic study of CHjS chemistry, using pulsed laser photolysis with pulsed LIF detection. We report here results on the reactions of C H S with 0 (1) and N 0 (2), which are potential removal mechanisms in the atmosphere.
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CH S + 0
— > products
(1)
CH S + N 0 — > products
(2)
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Experimental A l l experiments were carried out by pulsed 248 nm laser photolysis of CH3SSCH3 (DMDS) in excess bath gas to generate CH3S, followed by pulsed laser induced fluorescence detection of CH3S. OH, or NO. CH3S, O H and NO were detected by exciting the A ( A i ) in 100 torr 0 : its concentration was not large due to its fast reaction with DMDS (12). Figure 2 shows the dependence of tne pseudo-first order rate coefficients on the concentration of N 0 for experiments using He, N and 0 . The He and N data have been offset by 10 s* for clarity. We estimate a maximum uncertainty of 10% related to the measurement of flow rates and NOo concentration, leading to a preferred rate coefficient k = (6.1 ± 0.7) x 10* cm molec" s* . Our value for the rate coefficient of the reaction CH3S + N 0 is about 40% lower than that measured by Balla et al. (1.1 ± 0.1) x 10* cm molec* s* using a very similar technique (12). This group also found no dependence on pressure, and measured a weak negative temperature dependence, leading them to speculate that the reaction may proceed to give CH3SO + NO as products. We have measured NO produced in this reaction and found that the NO production had two temporally distinct components. The faster component could be correlated with CH3S + NO?, while the slower rise is due to the subsequent reaction of CH3SO with N 0 . We therefore suggest that under these conditions («10 N 0 cm" and < 1 torr 0 ) the following reactions occur: 3
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CH S + N 0 — > CH3SO + NO
(2)
CH3SO + N 0
(4)
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— > C H S 0 + NO.
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These reactions are analogous to those of HSO, as proposed by Lovejoy et al. (2Q). The NO data were fitted by a non-linear least squares routine to an analytical solution. This yielded values for k , the branching ratio of reaction (4) to give NO and the overall yield of NO. The fitted values were not definitive since some of the NO appears to be produced vibrationally excited, and relaxation may not have been complete on the time scale of the experiment. Values of 1-4 fell in the range (8 ± 4) x IO" cm molec* s* . The overall yield of NO produced was around 1.5 per CH S, and we suspect that the yields may actually be close to 1.0 for both reactions (2) and (4). Further experiments are in progress to elucidate the reaction sequence. More detailed accounts of both the 0 and N 0 reactions will be published shortly (21). 4
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The results of our experiments, along with those of Balla et al. (12) confirm the earlierfindingsof Grosjean (2) and Hatakeyama and Akimoto (4), that reaction of CH3S with 0 i s much slower than with NO or N O * Grosjean obtained Np2/*02 = 2 x 10 , and Hatakeyama k / k o 2 = 2 x 10 at 760 torr. These estimates lead to rate coefficients for CH S + 0 of 3 x IO* and 2 x IO* cm molec* s* respectively, based on the directly measured rate coefficients for the N 0 and NO (12) reactions. However, the mechanisms used in deriving both of these ratios were incomplete. Our results suggest that CH3S reacts very slowly with 0% if it reacts at all. The mechanism proposed by Balla and Heicklen, in which CH3S is regenerated by O H radicals, would lead to a detectable amount of O H in our system, if the rate coefficient were close to IO* cm molec* s* as their (corrected) result suggests. As stated earlier, no O H was detected and we could place an upper limit for this process which is two orders of magnitude lower than Balla and Heicklen's value. The efficient conversion of CH3S to S 0 in the C. W. photolysis experiments which used very low levels of N O points to the possible occurrence of a CH3S + 0 reaction in the atmosphere. Further experiments are required to assess 2
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Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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27. TYNDALL AND RAVISHANKARA
Atmospheric Reactions ofCH£ Radicals 455
Figure 2. Pseudo first order rate coefficient for CflUS + N 0 as a function of [NOJ. Data for He and N have been offset by 10 s' for clarity. Symbols used are: a He, 40 torr; • He, 100 torr; O 40 torr; + 85 torr; O O2, 40 torr; + 0 , * 85 torr. 2
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Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
456
BIOGENIC SULFUR IN THE ENVIRONMENT
this possibility. One explanation is that radical- radical reactions of CH3S could become important in experiments where SO2 production is observed under conditions of very low NO . x
CH S + H 0 — > CH3SO + O H 3
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Akimoto and coworkers detected S following 248 nm photolysis of DMDS, implying that C H radicals are also produced (22). Low levels of C H * 0 and H 6 radicals that could be produced in the C.W. experiments would not be removed by reactions with N O but rather sustain a chain process leading to S0 production. The upper limit for the C H S + 0 reaction rate coefficient determined here is an order of magnitude lower than previous estimates. Even with this lower value we still can not rule out this reaction in the atmosphere. 0 has a mixing ratio of 0.21 which implies a loss rate for CH3S < 12 s . Even though the N 0 reaction rate coefficient is 6 x 10" cm molec s , the reaction with 0 coula be dominant for N 0 concentrations less than 2 x 10 molec cm , or a mixing ratio of - 8 ppb. Since most CHjS is produced in the marine troposphere, where N 0 is typically < < lppb, the Oo reaction can not be ruled out. Since the C H ^ O ^ ) adduct, if it is formed at all, does not appear to react with O ^ our upper limit is the real upper limit for CH3S loss in air. Pertinent to this discussion is the possible occurrence of a reaction between C H 3 S and O3. Several recent studies have found that HS reacts with O3 with a rate coefficient of approximately 4 x 10* cm molec s (23.24). In view of the similarity of the rate coefficient for CH S with NO^ 6.1 x 10 cm molec s , with that for HS with N 0 , 6.7 x 1 0 cm molec s (25), it could be expected that C H 3 S reacts with O 3 with a rate coefficient in excess of 10 cm molec s . Black and Jusinski reported an upper limit for this reaction of 8 x 10 cm molec s , but the determination used very high (> 10 molec cm ) concentrations of O 3 (26). Since the potential for regeneration of CH S is very high in such a system (cf Friedl et al. (22) in the HS + O3 reaction), a reaction between CH S and O3 should not be excluded. The rate coefficient for C H 3 S with N 0 reported here is about 40% lower than that reported by Balla et al. While we observed a decrease of up to 30% in the value of the rate coefficient in 100 torr 0 , we believe this to be due to regeneration of CH^S in our N0 -rich system. However we feel that the experimental conditions were sufficiently well-controlled that we have eliminated systematic errors. There have recently been several measurements of the rate coefficient for the reaction of HS with N 0 (25 and references therein). Our value for CH3S + N 0 is very close to the rate coefficient for HS + N 0 determined by Wang etal. (25). Our results shed some light on the mechanism for the conversion of sulfides to S 0 and MSA in the atmosphere and in chamber experiments. If the N 0 (or an O3) reaction dominates, then C H 3 S will be converted to C H 3 S O : 2
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CH3SO, it seems, can further react with NO* or, as recently shown by Barnes et al.(2Z),add0 : 2
Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
27. TYNDALL AND RAVISHANKARA CH3SO + N 0
Atmospheric Reactions of CHjS Radicals 457
> CH3SO2 + NO
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CH3SO + 0 + M — > C H S ( 0 ) 0 + M 2
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Barnes et al. identified a product CH3S(0)0 N0 , suggesting that 0 can effectively compete with NO? for CH3SO at atmospheric pressure. It seems, therefore, that in systems with high N O (i.e., chamber experiments) formation of CH3SO and CH3SO? may predominate, and this could lead to the elevated levels of M S A detected in these systems. Hatakeyama et al. (28) have recently used isotopic substitution to show that some of the SO? formed comes from N 0 reaction with CH3S or CH3SO. Furthermore, at nigh N O the S 0 yield is independent of 0 or N (27.28). pointing strongly to the involvement of these radicals in the production of S0 . One of the outstanding problems remains, therefore, to measure the rate coefficients of reactions of CH3SO and CH3SO2 with, for example, NOp Op and O 3 , and to identify the products. One particularly interesting question is whether the production of CH3SO radicals in air at low N O mixing ratio leads to SO? or MSA formation. Under clean troposphere conditions, however, an oxidation chain initiated by CH3S + 0 could give different products, and it is essential to understand the mechanism of S 0 formation at low N O . It will be interesting to see whether the same radicals lead to S 0 formation in the high and low N u cases. Since the reaction of CH3S with 0 appears to be so slow, it may be necessary to perform C.W. photolysis studies under very carefully controlled conditions in order to determine whether a slow reaction actually occurs, or whether secondary reactions lead to the observed products. 2
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Acknowledgments This work was supported by N O A A as part of the National Acid Precipitation Assessment Program. Literature cited 1. Atkinson, R.; Pitts, J. N., Jr.; Aschmann, S. M . J. Phys. Chem. 1984, 88, 1584. 2. Andreae, M . O.; Ferek, R. J.; Bermond, F.; Byrd, K. P.; Engstrom, R. T.; Hardin, S.; Houmere, P. D.; LeMarrec, F.; Raemdonck, H.; Chatfield, R. B. J. Geophys. Res. 1985,90,12891. 3. Grosjean, D. Environ. Sci. Tech. 1984, 18, 460. 4. Hatakeyama, S.; Akimoto, H . J. Phys. Chem. 1983,87,2387. 5. Hatakeyama, S.; Izumi, K.; Akimoto, H . Atmos. Environ. 1985, 19, 135. 6. Saltzman, E . S.; Savoie, D. L.; Zika, R. G.; Prospero, J. M . J. Geophys. Res. 1983, 88, 10897. 7. Yin, F.; Grosjean, D.; Seinfeld, J. H . J. Geophys. Res. 1986, 91, 14417. 8. Toon, O. B.; Kasting, J. F.; Turco, R. P.; Liu, M . S. J. Geophys. Res. 1987, 92, 943. 9. Hynes, A. J.; Wine, P. H . J. Phys. Chem. 1987,91,3672.
Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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10. Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Int. J. Chem. Kin. 1983, 15, 647. 11. MacLeod, H.; Jourdain, J. L.; Poulet, G.; LeBras, G. Atmos. Environ. 1984, 18, 2621. 12. Balla, R. J.; Nelson, H . H.; McDonald, J. R. Chem. Phys. 1986, 109, 101. 13. Balla, R. J.; Heicklen, J. J. Photochem. 1985, 29, 297. 14. Barnes, I.; Bastian, V.; Becker, K. H . ; Fink, E. H . In Physico-Chemical Behaviour of Atmospheric Pollutants; Angeletti, G ; Restelli, G., Eds.; Fourth European Symposium: Reidel, Dordrecht, 1987; p 327. 15. Wahner, A.; Ravishankara, A. R. J. Geophys. Res. 1987,92,2189. 16. Black, G.; Jusinski, L. E. J. Chem. Phys. 1986, 85, 5379. 17. Tyndall, G. S.; Ravishankara, A. R., in preparation. 18. Graham, D. M . ; Mieville, R. L.; Pallen, R. H.; Sivertz, C. Can. J. Chem. 1964, 42, 2250. 19. Wine, P. H.; Kreutter, N. M.; Gump, C. A.; Ravishankara, A . R. J. Phys. Chem. 1981, 85, 2660. 20. Lovejoy, E. R.; Wang, N. S.; Howard, C. J. J. Phys. Chem. 1987, 91, 5749. 21. Tyndall, G. S.; Ravishankara, A. R., J. Phys. Chem. 1988, submitted. 22. Suzuki, M.; Inoue, G.; Akimoto, H. J. Chem. Phys. 1984, 81, 5405. 23. Friedl, R. R.; Brune, W. H.; Anderson, J. G. J. Phys. Chem. 1985, 89, 5505. 24. Schönle, G.; Rahman, M . M.; Schindler, R. N . Ber. Bunsenges. Phys. Chem. 1987,91,66; Schindler, R. N.; Benter, T. ibid 1988,92,588. 25. Wang, N. S.; Lovejoy, E. R.; Howard, C. J. J.Phys.Chem. 1987,91,5743. 26. Black, G.; Jusinski, L. E. J. Chem. Soc.Faraday2 1986, 82, 2143. 27. Barnes, I.; Bastian, V.; Becker, K. H.; Niki, H . Chem. Phys. Lett. 1987,140, 451. 28. Hatakeyama, S; Akimoto, H., proceedings, this symposium. RECEIVED September 6, 1988
Saltzman and Cooper; Biogenic Sulfur in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
